This invention relates to web forming machines, such as Fourdrinier type paper machines wherein a pulp and water slurry is deposited onto a moving drainage or forming wire, and more particularly relates to drainage foils which are positioned in stationary supporting engagement beneath the moving drainage wire for purposes of increasing the drainage of water through the wire.
Such foils are well known and include a transverse leading edge which doctors or scrapes water from the underside of the moving wire, a land portion over which the wire passes, and a foiling portion which extends in a downstream direction from the land portion and at a divergent angle to the wire. The action of the foiling portion is to produce a suction, beneath the moving wire, which draws water through the wire. The leading edge of the adjacent downstream foil then scrapes this water from the underside of the wire. The suction force along the foiling portion causes the moving wire to be urged downwardly thus increasing the drag of the wire moving over the land portion. This increased drag causes rapid wear of the land portion thereby increasing the area of the land portion and moving the foiling area in a down-stream direction towards the trailing edge. Eventually, after prolonged wear, the increased area of the land portion will result in reduction in the area of the foiling portion to the point where the effective suction and drainage action is impaired. Additionally, the land portion wears irregularly across the width of the machine, which again causes unsatisfactory operation.
U.S. Pat. No. 3,165,440 - Jordansson - proposes one solution to the problems resulting from this kind of wear, which is to reverse the foil on its support. U.S. Pat. No. 3,446,702 - Buchanan - proposes another solution, which is to fit the foil with a wear insert, the object being to provide foils which have a high degree of resistance to wear. The present invention is directed to foils of the latter kind. Others have given attention to providing such foils, and the results have taken various forms, as disclosed for example in U.S. Pat. Nos.: 3,393,124 - Klinger et al; 3,778,342 - Charbonneau; 3,352,749 - Perry; 3,793,140 - Corbellini; and 3,105,789 - Goddard.